This invention is related to a method and apparatus for reducing jitter accumulation in a series of repeating devices along a physical communication channel. Jitter refers to the phase instability of a communication signal traversing a physical communication channel. Jitter is undesirable for systems in which phase stability and data synchronization of repeated data is desired (i.e., token ring local area network).
A data stream is received and retransmitted along a physical communication channel by a repeating device. To generally synchronize data transmission along the channel, the repeating device may, for example, derive the output clocking rate from the clock signal for the incoming data stream. According to such an approach, however, the communication as forwarded from repeating device to repeating device may accumulate jitter. The jitter appears as phase variations among the data pulses forming the communication. During successive repeating operations, jitter may accumulate beyond acceptable tolerances. Excessive jitter may cause the communication signal to be inaccurately recognized by a repeating device or network component. Accordingly, there is a need for reducing jitter accumulation.
A basic function of a repeating device is to receive and retransmit a communication signal along a physical communication channel. As the communication signal traverses the physical communication channel from a source, the signal loses strength. Correspondingly, the data pulses forming the signal attenuate and become distorted. Such attenuation and distortion characteristics limit the physical length of a communication channel. In practice, the limit depends upon the type of physical communication channel (i.e., twisted-wire-pair, coaxial, fiber-optic) and the type of transmission technique (i.e., baseband, broadband).
To enable the communication channel to extend beyond the limited distance and avoid the corresponding attenuation and distortion problems, a device for repeating the signal is used. The repeating device receives the communication signals, then retransmits the signals at substantially the original strength and sharpness. A conventional Repeater simply receives the signal, then retransmits the improved signal without processing or modifying the signal contents. Such a repeater is common for inter-continental cable transmissions.
FIG. 1 shows a block diagram of a conventional repeater 10 including a receiver 12, a data recovery circuit 14, buffer 16 and a transmitter 18. The data stream is input to the receiver 12, with the component pulses recognized by the data recovery circuit 14 and stored in the buffer 16. The transmitter 18 then retransmits the component data pulses at a restored amplitude and sharpness.
A repeating device, also may be a station on a network which receives the communication signals, then retransmits the signals at substantially the original strength and sharpness. For example, according to the standard protocol of a token ring LAN (IEEE standard 802.5), a communication referred to as a packet is passed from one station to the next station in physical sequence along a ring of stations. Each station transmits the packet to the next station regardless of the packet destination. Thus, each station is a repeating device. A station in a token ring network which provides the function of a repeating device includes the components 12, 14, 16, 18 of a conventional repeater 10 to receive, recognize and retransmit the communication.
The station embodiment of a repeating device may or may not modify the packet contents. According to the token ring architecture, a destination station sets status bits to acknowledge that the destination address was recognized and/or that the data was copied. In addition, any station in the ring may mark the packet with a transmission error bit.
Accordingly, a repeating device includes a device which receives a communication signal over a physical communication channel, then retransmits the signal with or without processing the signal contents.
The jitter tolerance of a repeating device limits the number of repeating devices which may be connected in series. For a token ring architecture in which each station is a repeating device, jitter is a significant problem. Accordingly, there is a need for substantially reducing and/or eliminating jitter from a series of repeating devices.
One conventional device for reducing jitter is a jitter attenuator. FIG. 2 shows a block diagram of a conventional jitter attenuator 20. The jitter attenuator 20 has a data input terminal, a data output terminal, a clock input terminal and a clock output terminal. The attenuator 20 includes a FIFO 22 which receives input data at an input clock data rate. A circuit 24 detects when the FIFO 22 is half-full. A detection signal output from circuit 24 is coupled to a variable oscillator 26 which adjusts the output clock frequency so as to keep the FIFO 22 half full. In effect, the output clock frequency is controlled to be the approximate average of the input clock frequency. As a result, the jitter of the output data signal is attenuated. Because the source signal (e.g., the input clock signal) for deriving the output clock signal has jitter, the output clock signal, although an average of such source signal, also includes jitter. Accordingly, there is a need for alternative methods and apparatus for reducing jitter, so as to better reduce or substantially eliminate jitter.